feat(security): seed Yerin agent + registry-selectable MCP server

- migration 011_yerin.sql: seed read-only 'yerin' agent ({read:true}, kind claude,
  model NULL = server default; switch to local Ollama via agents.model anytime)
- companion-stdio.js: select the toolset from VOID_TOOL_REGISTRY ('security' →
  Yerin's securityRegistry; default → Dross's companionRegistry)
- tests/mcp/registry_select.test.js

Remaining for Yerin (left for review): an entry point (route or cron) + persona
prompt — see docs/yerin-security-agent.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/mcp/registry_select.test.js

@@ -0,0 +1,40 @@
+import { describe, it, expect, beforeAll } from 'vitest';
+import { pool } from '../../lib/db/pool.js';
+import { resetDb } from '../helpers/db.js';
+import { migrateUp } from '../../lib/db/migrate.js';
+import { listMcpTools, callMcpTool } from '../../lib/mcp/companion-stdio.js';
+
+// The stdio MCP server must be able to expose Yerin's securityRegistry instead
+// of Dross's companionRegistry, selected by VOID_TOOL_REGISTRY at launch.
+
+describe('MCP registry selection', () => {
+  it('defaults to the companion registry when VOID_TOOL_REGISTRY is unset', () => {
+    const names = listMcpTools({}).map(t => t.name).sort();
+    expect(names).toEqual(['context', 'propose_change', 'read', 'search']);
+  });
+
+  it('selects the security registry when VOID_TOOL_REGISTRY=security', () => {
+    const names = listMcpTools({ VOID_TOOL_REGISTRY: 'security' }).map(t => t.name).sort();
+    expect(names).toContain('audit_log');
+    expect(names).toContain('agent_inventory');
+    expect(names).not.toContain('propose_change'); // Yerin cannot propose mutations
+  });
+
+  it('callMcpTool routes to the selected registry', async () => {
+    await resetDb(); await migrateUp();
+    const out = await callMcpTool('agent_inventory', {}, {}, { VOID_TOOL_REGISTRY: 'security' });
+    expect(Array.isArray(out.agents)).toBe(true);
+    // an unknown tool for the default registry must not be reachable in security mode
+    await expect(callMcpTool('propose_change', {}, {}, { VOID_TOOL_REGISTRY: 'security' }))
+      .rejects.toThrow(/unknown tool/i);
+  });
+});
+
+describe('migration 011 seeds Yerin', () => {
+  it('creates a read-only yerin agent', async () => {
+    await resetDb(); await migrateUp();
+    const { rows: [y] } = await pool.query(`SELECT * FROM agents WHERE slug='yerin'`);
+    expect(y).toBeTruthy();
+    expect(y.capabilities).toEqual({ read: true, suggest: false, write: false });
+  });
+});